In seismic prospecting as contemplated herein, there is provided a source of energy, typically sonic energy, which is generated adjacent the surface of the earth or of a body of water in offshore prospecting. The generated energy travels through the earth or water medium to discontinuities and interfaces among various strata thereof. At each such discontinuity or interface, there is provided both a reflected and a diffracted seismic wave. These waves in turn return to the surface and are detected by a plurality of seismic transducers, or geophones, which may be arranged along a seismic line having a particular geographic orientation. The magnitude and phase displacement between the returning signals and the originally generated signals are indicative of the displacement between the source of the seismic signals and the reflecting and/or diffracting interface. The data obtained by a plurality of linear arrangements of such transducers may be used to map the various interfaces and discontinuities.
For offshore prospecting, wherein a plurality of geophones may be spaced fairly closely to one another (approximately 82 feet apart, for example) along a single seismic line, the various lines are typically placed on the order of one-half mile to one mile apart from one another. Thus, although return data are relatively dense in a longitudinal direction determined along the seismic lines of geophones, these data are nonetheless relatively sparse in the direction transverse thereto. Any determinations of dip and/or strike in the transverse direction are thus subject to error because of the relatively large transverse discontinuities between transducers.
In order to provide more accurate representation of subsurface geological structures, more detailed sampling of the subsurface areas are required. Such increased detail is available by providing more dense, three dimensional, seismic coverage over the exploration area, rather than standard two dimensional data collected by individual lines of receives to provide a profile representation of the surveyed structures. In the three dimensional seismic prospecting process, the seismic lines of transducers are repositioned at substantially closer transverse intervals to one another than in the conventional approach.
By obtaining the more densely and uniformly packed seismic data, the spurious assumption that the surveyed geologic structure is two dimensional is no longer required. Thus, the received data are manipulated and processed as a true three dimensional wave field, in accordance with correct physical principles. With such processing, the received seismic energy is better imaged to reveal the subsurface geology than the two dimensional representations thereof. Additionally, the availability of three dimensional data permits an interpreter to view the surveyed structure in a three dimensional perspective, either through a time-slice of the seismic data or by obtaining a vertical slice at any orientation. Such flexibility in display makes the seismic interpretation easier, faster and more accurate.
However, because of the time consumptive nature of the seismic prospecting process, it is substantially more expensive to obtain such more highly densely packed three dimensional data than it is to obtain the standard (two dimensional) prospecting data. There is accordingly a need in the prior art to provide a method and apparatus for obtaining three dimensional, densely packed, seismic prospecting data in a less expensive manner, thereby to enable more accurate seismic prospecting of geologic structures.
More specifically, it is desirable to be able to regrid existing two dimensional data in a manner to provide three dimensional seismic data, thereby enabling large quantities of data to be evaluated quickly and accurately. A number of unsuccessful attempts have been made in the prior art to regrid such two dimensional data with the aid of varying computational techniques. In addition to requiring seismic data obtained by perpendicular seismic lines, a further significant problem with the prior art is the inability of such regridding programs to function for structures which deviate from a very flat configuration between the points mapped by the two dimensional technique. There is thus a need in the prior art to provide a method for regridding two dimensional data to provide a true three dimensional representation of the surveyed structure.
It is accordingly an object of the present invention to overcome the difficulties of the prior art and to provide three dimensional, or densely packed, seismic data in a seismic exploration process.
It is a more specific object of the invention to provide such three dimensional seismic prospecting data without requiring a more dense and expensive positioning of various geophones for receiving the reflected and diffracted seismic signals.
It is another object of the invention to provide a method and apparatus for converting sparse, two dimensional, data representative of a geological structure or surface to more accurate and densely packed three dimensional data representative thereof.
It is still another object of the invention to obtain a mathematical representation of the geologic structure being surveyed by performing operations on sparse, two dimensional, seismic data signals representative thereof.
An additional object of the invention is the conversion of two dimensional seismic data to three dimensional, more densely packed, data representative of a geologic surface corresponding to the surveyed structure.
Still another object of the invention is the provision of data descriptive of a geologic structure, obtainable at any arbitrarily selected point with respect to said structure, from previously obtained two dimensional seismic prospecting data representing said structure.
Yet a further object of the invention is to provision of data obtainable by a seismic line, disposed at an arbitrary orientation with respect to a geologic surface, from standard two dimensional prospecting data descriptive of the structure.
It is a further object of the invention to convert sparse, two dimensional, seismic data descriptive of a geologic structure to dense, three dimensional, data descriptive of the structure without the necessity of placing additional lines of transducers or of obtaining additional data.
Yet another object of the invention is the provision of regridded three dimensional seismic data from existing two dimensional data representing a geologic structure.